The operation of combustion devices and particularly internal combustion (IC) engines produces small amounts of air contaminants such as carbon monoxide (CO), nitrogen oxides (NO, NO2, or generally NOx), hydrocarbons (HC) and ammonia in exhaust gas. Carbon monoxide, NOx, and ammonia are problematic air pollutants and their emission levels are regulated within certain limits in many geographical regions. NOx emissions are of particular concern. In some areas, significant NOx reductions are needed in order to meet National Ambient Air Quality Standards (NAAQS). Ammonia is a regulated toxic air contaminant and a precursor for the formation of particulate matter. Some regulatory agencies assess fees for certain ammonia emissions. Equipment with ammonia emissions can be subject to Best Available Control Technology (BACT) requirements and Best Available Control Technology for Toxics (TBACT) requirements. Particulate matter is an air contaminant and a carcinogen.
For smaller engines (less than 1000 HP), commonly adopted exhaust gas treatment post-combustion involves a single stage catalyst system. In some areas, it is not unusual for engines with exhaust treatment systems employing single stage catalysts to frequently exceed their regulated emission limits. For an engine to be in compliance, the air to fuel ratio must be within a narrow range. Air to fuel ratio controllers (AFRCs) control the air to fuel ratio with the use of an oxygen sensor. However, oxygen sensors can be affected by a number of factors such as exhaust temperature, engine load changes, unburned methane in the exhaust, ambient humidity, oxygen sensor age and other factors that might affect oxygen sensor output. Oxygen sensor “drift” or incorrect oxygen sensor signal is a major cause for noncompliance with emission standards. Oxygen sensor “drift” is a particular problem for engines that operate at variable load.
Some engines employ Selective Catalytic Reduction (SCR) systems to control emissions. These systems have a number of disadvantages, especially for smaller engines. SCR catalysts usually contain large quantities of rare precious metals and require regular replacement. SCR systems require the storage, handling and pumping of hazardous chemicals such as ammonia or urea which can subject the corresponding facilities to stringent regulatory requirements. Therefore, a system to control dosing of ammonia or urea is needed. Ammonia emissions from engine systems with SCR are frequently regulated. Inappropriate dosing can lead to toxic ammonia emissions exceeding regulatory limits. Further, the SCR systems can require frequent maintenance and specialized technicians.
Previously, there was experimentation with two stage catalyst systems involving injection of air between the two catalyst stages. Despite the complexity of two catalysts, these systems did not result in markedly improved emission control and, in fact, the two stage catalyst system could result in generation of NOx on or in communication with the second stage catalyst.